The present invention generally relates to a GPS (Global Positioning System) reception method and a GPS receiver.
In GPS systems for obtaining positions of mobile bodies by use of artificial satellites (hereafter referred to as GPS satellites), the basic functions of each GPS receiver are that the receiver receives signals from four or more GPS satellites, computes the current position of its position from the received signals, and let the user know the computed position.
The GPS receiver demodulates the signals received from GPS satellites (hereafter, these signals are referred to as GPS satellite signals) to obtain the orbit data of the GPS satellites and, from the each GPS satellite's orbit, time information and delay time, derives the three-dimensional position of the own receiver on the basis of simultaneous equations. The signals from four GPS satellites are necessary for positioning computation because the effects of the error between the time in the GPS receiver and the time of each satellite must be eliminated.
In the case of consumer GPS receivers, the signal radio spectrum-spread by a spread code called L1 band C/A (Clear and Acquisition) transmitted from GPS satellites (Navstar) is received for positioning computation.
The C/A code is a code of a PN (pseudo random noise) sequence having a transmission signal rate (or chip rate) of 1.023 MHz and a code length of 1023, a spread code consisting of Gold codes, for example. The signals transmitted from the GPS satellites are each a signal obtained by executing BPSK (Binary Phase Shift Keying) on the carrier having a frequency of 1575.42 MHz by a signal obtained by spectrum-spreading data of 50 bps by use of a spread code. In this case, the code length is 1023, so that, in the C/A code, the PN sequence code repeats in one period of 1023 chips (therefore, 1 period=1 millisecond) as shown in FIG. 34A.
The PN sequence code of the C/A code is different from one GPS satellite to another. However, each GPS receiver can detect beforehand which PN sequence code is used by which GPS satellite. In addition, a navigation message (orbit information) to be described later allows the GPS receiver to know which GPS satellite's signal it can receive at its position and at that point of time. Therefore, in the case of three-dimensional positioning, the GPS receiver receives radio waves from four or more GPS satellites that can be acquired at that position and at that point of time, spectrum-despreads the received signals, and executes a positioning computation on the basis of the despread signals, thereby obtaining its own position.
As shown in FIG. 34B, one bit of satellite signal data (or navigation message data) is transmitted as 20 periods of the PN sequence code, namely on a 20 milliseconds basis. That is, the data transmission rate is 50 bps. The 1023 chips for one period of the PN sequence code is inverted between the bit being “1” and “0”.
As shown in FIG. 34C, 30 bits (600 milliseconds) form 1 word in GPS. As shown in FIG. 34(D), 10 words form 1 subframe (6 seconds). As shown in FIG. 34(E), the start word of each subframe is always inserted with a preamble that is a predetermined bit pattern even if the data is updated. This preamble data is followed by data.
Five subframes form one main frame (30 seconds). The navigation message is transmitted in units of data of this 1 main frame. The three subframes of the data of this 1 main frame provide satellite-unique orbit information called ephemeris information. This ephemeris information is transmitted in a repetition of 1 main frame (30 seconds) and includes the parameters for obtaining the orbit of the satellite that transmits this information and the transmission time of the signal from the satellite.
Namely, the second word of the three subframes of ephemeris information includes TOW (Time Of Week) and the third word of the first subframe 1 of the main frame includes the time data called Week Number. The Week Number is the information that is counted up every week with Jan. 6 (Sunday), 1980 being week 0. Also, TOW is the information that is counted up every 6 seconds (namely, every period of subframe) with 0:00 of the Sunday being 0.
Each of the GPS satellites has an atomic clock to use the common clock data and the time at which the signal is transmitted from each GPS satellite is synchronized with the atomic clock. The absolute time is obtained by receiving the above-mentioned two clock data. Any value below 6 seconds is synchronized with the time of the satellite in the process of sync-locking to the radiowave of the satellite with the accuracy of the reference oscillator of that GPS receiver.
Also, the PN sequence code of each GPS satellite is generated as synchronized with the atomic clock. The position and velocity of the satellite for use in the positional computation in the GPS receiver are obtained from this ephemeris information.
The ephemeris information is a precision calendar that is updated comparatively and frequently under the control of the ground control station. By holding this ephemeris information in the memory, the GPS receiver can use it for positional computation. However, the service life of the ephemeris information is normally about two hours in terms of accuracy, so that the GPS receiver monitors the time from the moment at which the ephemeris information is stored in the memory and, when its service life has exceeded, updates and rewrites the ephemeris information stored in the memory.
It should be noted that it takes at least 18 seconds (equivalent to three subframes) to update the contents of the memory with the ephemeris information newly obtained from the GPS satellite and it takes consecutive 30 seconds if the data is obtained halfway between subframes.
The orbit information of the remaining two subframes of the data of one main frame is the information called almanac information that is commonly transmitted from all satellites. The 25 frames of the almanac information are required to obtain all information. The almanac information is composed of the information indicative of the approximate position of each satellite and the information indicative of which satellites are available.
This almanac information is also updated at least once every few days under the control of the ground control information. The almanac information can be stored in the memory of the GPS receiver for use. The service life of the almanac information is several months. With time, the accuracy of the position determination of the satellite lowers, but the almanac information remains useful enough for recognizing the approximate position of the satellite. Normally, the almanac information is updated while the GPS receiver is being used. Storing the almanac information in the memory of the GPS receiver allows, upon powering on the receiver, the computation for which satellite is to be allocated to which channel.
In order to obtain the above-mentioned data by receiving the GPS satellite signal by the GPS receiver, the same PN sequence signal (hereinafter, the PN sequence spread code is referred to as the PN code; the PN sequence spread code of the GPS satellite is referred to as the satellite PN code; and the corresponding PN sequence spread code of the GPS receiver is referred to as the replica PN code) as the C/A code used on the GPS satellite to be received, the PN sequence code being prepared on the GPS receiver, is used to acquire the GPS satellite signal by phase-synchronizing the C/A code for that GPS satellite signal, thereby executing spectrum despreading. When the phase synchronization with the C/A code has been successful for despreading, bit detection is executed to allow the acquisition of a navigation message including time information and so on from the GPS satellite signal.
The GPS satellite signal is captured by C/A code phase synchronization search. In this phase synchronization search, a correlation between the replica PN code of the GPS receiver and the satellite PN code of the GPS satellite is detected and, if the obtained correlation value is greater than a predetermined value, the synchronization between both is determined established. If no synchronization is found established, the phase of the replica PN code of the GPS receiver is controlled by use of some synchronization method to synchronize the replica PN code with the satellite PN code.
As described above, the GPS satellite signal is obtained by BPSK-modulating the carrier with a signal obtained by spreading data with the satellite PN code, so that, for the GPS receiver to receive the GPS satellite signal, synchronization must be established not only between the PN codes but also between the carrier and the data. However, the synchronization of the PN codes the carrier cannot be executed independently.
The GPS receiver, it is a normal practice to convert the carrier frequency of each received signal into an intermediate frequency within several MHz and execute the above-mentioned synchronization detection processing on the received signal in the state of this intermediate frequency signal. The carrier frequency of this intermediate frequency signal (namely, the intermediate frequency carrier frequency) includes a frequency error caused by the Doppler shift corresponding to mainly the moving velocity of the GPS satellite and a frequency error component of the local oscillator generated inside the GPS receiver when a received signal is converted into an intermediate frequency signal. The frequency error component of the local oscillator included in this intermediate frequency signal is hereafter referred to as an intermediate frequency carrier error.
Now, let the intermediate frequency carrier frequency of a received signal be fIF, a predetermined intermediate frequency carrier frequency be FIF, the Doppler shift of the GPS satellite be fD, and the intermediate frequency carrier error be ΔfIF, then the above-mentioned intermediate frequency carrier frequency f is expressed in equation below.fIF=FIF+fD+ΔfIF  (equation a)
Due to the above-mentioned frequency error factor, the carrier frequency in the intermediate frequency signal is unknown; there it is necessary to establish IF carrier synchronization by executing frequency search. Also, because the synchronized point (or the synchronized phase) of the PN code within one period depends on the positional relationship between the GPS receiver and the GPS satellite, the synchronized point is unknown, so that some synchronization method is required to establish the synchronized point as described above.
If it takes time for the synchronization between the spread code and the IF carrier, the response of the GPS receiver is delayed, thereby presenting a problem of inconvenience in the use of the GPS receiver.
With related-art GPS receivers, the synchronization between the carrier and the spread code is detected by the sliding correlation involving frequency search and, at the same time, synchronization acquisition and hold operations are executed by means of DLL (Delay Locked Loop) and Costas loop. However, the synchronization acquisition by sliding correlation and the synchronization hold by DLL and Costas loop are not suitable for a high-speed synchronization acquisition in principle, so that, with actual GPS receivers, the processing up to synchronization acquisition is shortened by use of a multi-channel configuration.
Patent Document (Japanese Patent Laid-open No. 2003-258969) discloses a configuration in which the synchronization acquisition section and the synchronization hold section are separated from each other, the synchronization acquisition section is constituted by a matched filter, and the synchronization hold section is constituted by DDL and Costas loop, thereby executing synchronization acquisition and synchronization hold operations at high speeds.
The above-mentioned patent document is as follows.
[Patent Document 1]
Japanese Patent Laid-open No. 2003-258969
Execution of positioning computation with a GPS receiver requires at least the position of the satellite and the range between the satellite and the receiver. The position of the satellite can be obtained from the ephemeris information of the orbit information described above.
The range between the GPS satellite and the GPS receiver can be computed by the GPS receiver by measuring the period of time in which a signal transmitted from the satellite at a certain time reaches the GPS receiver (namely, the signal arrival time=the difference between the time at which a spread code is originated and the time at which the spread code arrives) and multiplying the obtained time by the velocity of light (3×108 m/s). However, the above-mentioned range computed by the GPS receiver contains an error due to a clock error for example between the GPS receiver and the GPS satellite, so that this range is generally referred to as a pseudo range.
With the GPS receiver, the amount of the influence of the clock error to the pseudo range is also unknown, so that the number of unknown values to be computed is 4, this unknown amount plus the above-mentioned unknown values three-dimensional coordinates. Therefore, the GPS receiver captures the radio waves from the four satellites to execute three-dimensional positioning. Currently, 32 GPS satellites are available for example, the GPS receiver selects a set of four satellites that are available and produces less error, thereby executing three-dimensional positioning. The following equations are used for this three-dimensional positioning:r1={(x1−X)2+(y1−Y)2+(z1−Z)2}1/2−sr2={(x2−X)2+(y2−Y)2+(z2−Z)2}1/2−sr3={(x3−X)2+(y3−Y)2+(z3−Z)2}1/2−sr4={(x4−X)2+(y4−Y)2+(z4−Z)2}1/2−swhere,
ri (i=1, 2, 3, 4): pseudo range of GPS satellite i;
X, Y, Z: position of GPS receiver;
xi, yi, zi: position of GPS satellite i; and
s: the amount of influence of the clock error of the GPS receiver to range.
Each of the above-mentioned equations is a quadratic having no multiplication term between different unknown values; generally, simultaneous equations are solved by a method of iteration such as Newton method with an appropriate initial value near the solution given. In Newton method, a given equation is locally linearly approximated at a point near the solution, the linear simultaneous equations are solved by use of an initial value, the result of the solution is used as a next initial value to obtain the solution, this operation is repeated until the solution falls in a certain error range, thereby obtaining the final solution.
In environment in which the received signals from four or more GPS satellites can be obtained with stability, the GPS receiver can obtain its own velocity as follows. Namely, first, for each of the four or more GPS satellites, the GPS receiver computes the satellite position and velocity from the orbit information and the origination time of the spread code. Next, by use of the position of the GPS receiver obtained as described above and the carrier frequency of the received signal from the GPS satellite obtained by synchronization hold, linear simultaneous equations are set up with the three-dimensional velocity of the GPS receiver and the intermediate frequency carrier error given as unknown values. By solving these linear simultaneous equations, the velocity of the GPS receiver can be obtained. The error of the intermediate frequency carrier obtained this time is an error good in accuracy in the receiving environment and can be used to determine the intermediate frequency carrier frequency in capturing a new GPS satellite signal.
In order to minimize the period of time from powering on of the GPS receiver to the obtaining of the position of the GPS receiver, many GPS receivers retain, in their internal memory area also when the power to them is off, the orbit information of the GPS satellite from which a signal had been received immediately before the powering off and the three-dimensional coordinates and the intermediate frequency carrier error of the GPS receiver at the time the last positioning was made.
If the intermediate frequency carrier frequency of the GPS satellite is known in acquiring the synchronization of a GPS signal, it is expected to execute synchronization acquisition in a shorter time. Intermediate frequency carrier frequency f can be obtained from (equation a) mentioned above.
In (equation a), intermediate frequency FIF is a predetermined value. Doppler shift fD of the GPS satellite can be computed from the position and velocity of the GPS receiver and the position and velocity of the GPS satellite. If the velocity of the GPS receiver is unknown, it is assumed to be 0 and an approximate value of Doppler shift fD of the GPS satellite can be obtained from the information of the internal clock (namely, the RTC (Real Time Clock)) of the GPS receiver and the three-dimensional coordinates of the GPS receiver and the orbit information of the GPS satellite stored in the internal memory area of the GPS receiver.
Consequently, if intermediate frequency carrier error Δf is known, the intermediate frequency carrier frequency can be obtained from (equation a). Therefore, it is expected that, with the GPS receiver in the powered state and the four GPS satellites not yet in the acquired state, the synchronization acquisition can be made in a short time by use of intermediate frequency carrier error Δf obtained immediately before the last power-off and stored in the memory of the GPS receiver.
However, the oscillator that provides reference to the local oscillation frequency that is directly involved in the intermediate frequency carrier error Δf may vary from several tens of Hz to several hundred Hz during a period of time from several seconds to several tens seconds depending on various conditions such as ambient temperature and vibration in addition to individual specificity of the oscillator.
Hence, even if the period of time in which the power is off for several seconds to several tens of seconds, the intermediate frequency carrier error value stored in the memory area in the GPS receiver may be offset by approximately several tens of Hz to several hundred Hz until updated to the value in that use condition due to the computation of the velocity of the GPS receiver after it is powered on as described above.
Consequently, if the synchronization acquisition of the received signals from the four or more GPS satellites is attempted by use of the above-mentioned intermediate frequency carrier error without any change, stored when the power was turned off, a problem occurs that it takes time to execute the synchronization acquisition because four or more GPS satellite signals must be synchronously acquired in a frequency range considering a variation of approximately several tens of Hz to several hundred Hz depending on the above-mentioned conditions such as ambient temperature and vibration.